1,000 light bulbs with a mean life of 120 days are installed in a new fac- tory. Their length of life is normally distributed with standard deviation 20 days. () How many bulbs will expire in less...

Extracted text: 1,000 light bulbs with a mean life of 120 days are installed in a new fac- tory. Their length of life is normally distributed with standard deviation 20 days. () How many bulbs will expire in less than 90 days ? (i) If it is decided to replace all the bulbs together, what iniervals should be allowed between replacements if not more than 10 per- cent should expire before replacement ?